1. Field of the Invention
This invention relates to a method for forming a thin film having abrasion resistance and water repellency mainly on the surface of a resin product, and, in particular, to a method of forming an abrasion-resistant and water-repellent thin film on the surface of a transparent resin glass.
2. Description of the Related Art
The windowpanes of automobiles, etc. are generally formed of inorganic glass products. Recently, however, use of transparent resin products, which are lightweight and have excellent workability, in place of inorganic glass products, has been proposed. Further, the use of resin products, which have excellent formability, instead of metal products, has been proposed for such mechanical components as gears. However, resin products are inferior to inorganic glass products and metal products in abrasion resistance, and so there are limitations in their use. In view of this, various thin-film forming methods for forming an abrasion-resistant thin film on the surface of a resin product have been considered.
For example, in a known method, an uncured layer consisting of a methacrylic or acrylic-type UV-polymerizing curable coating is formed on a clean resin substrate, and irradiated with UV (ultraviolet rays) to effect polymerization, thereby forming a cured thin film. This method is convenient in that the resin substrate and the methacrylic or acrylic-type UV-polymerizing curable coating firmly adhere to each other, so that it is possible to form a thin film on various resin substrates without providing a primer layer therebetween. However, the above thin-film forming method has a problem in that the thin film formed does not have a sufficient degree of abrasion resistance since commercially curable compositions that are capable of forming a thin film having a sufficient degree of hardness are not yet available.
In another known thin-film forming method, a silicone-type thermally polymerizing curable coating, which is a typical thermally polymerizing curable coating, is used. According to the known method, a primer layer for achieving an improvement in adhesion is applied to a resin substrate, and, after curing this layer by drying, an uncured layer consisting of a silicone-type thermally polymerizing curable coating is formed on the primer layer. Next, the coating is thermally cured by a heating process. In this known method, a thin film is formed of a silicone-type thermally polymerizing curable coating, so that a large amount of silicon oxide exists on the surface, thereby providing excellent abrasion resistance. However, since a primer layer has to be provided between the thin film and the substrate in order to improve the adhesion of the thin film, an increase in the number of processes is entailed. In particular, when the resin substrate is formed of an engineering plastic like polycarbonate, it is impossible for the thin film to adhere without providing a primer layer therebetween.
When using a transparent resin plate as an automotive windowpane, it is desirable that the plate exhibit water repellency in addition to abrasion resistance.
From the viewpoint of water repellency only, it is possible for to ensure a contact angle of 75 between the resin plate and water if the resin substrate is formed of PC (polycarbonate) due to the hydrophobic property of the substrate itself. However, a contact angle of this degree is not sufficient to provide the requisite water repellency. Thus, when used as an automotive windowpane or the like, it is difficult for this resin substrate to ensure satisfactory visibility in rainy weather or the like. Further, this resin substrate does not satisfy the requirements in terms of abrasion resistance. Also, when the above commercial water-repellent agent is simply applied to the resin substrate, the resultant water-repellent layer is rather poor in terms of abrasion resistance.
When a thin film consisting of an acrylic curable coating is formed on a resin substrate as described above, the contact angle it makes with water is approximately 60.degree. due to the hydrophobic property of the acrylic groups, so that it is possible to obtain some degree of water repellency. However, a contact angle of this degree cannot be regarded as sufficient. Further, the thin film does not provide satisfactory abrasion resistance.
Similarly, in the case of a thin film consisting of a silicone-type curable coating formed on a resin substrate through the intermediation of a primer layer, the contact angle the thin film makes with water is increased to approximately 82.degree. due to the influence of the Si(CH.sub.3) bond, leveling agent, etc., thus providing water repellency to some degree. Further, due to the structure of SiO.sub.2-x (x=0.6-0.7), it is possible to obtain a measure of abrasion resistance. However, a contact angle of this degree is still insufficient to provide the requisite water repellency. In particular, the surface-active agent, leveling agent, etc. degenerate as a result of exposure to ultraviolet rays or the like during use, thereby rendering it impossible for the thin film to reliably provide a sufficient degree of water repellency.
Distinct from the above, a method has been developed according to which a water-repellent and abrasion-resistant thin film consisting of SiO.sub.2 containing fluorine is formed by the sol-gel method on an inorganic glass plate generally used for automotive windowpanes (see, for example, "Preliminary Reports of Annual Lectures", Ceramic Society of Japan, Vol. 199 1, page 96, or "Journal of Non-Crystalline Solids 121 (990) 344-347 North-Holland").
It might be possible to apply this sol-gel method to a resin plate, forming a water-repellent and abrasion-resistant thin film on the resin plate. However, as stated in the above-mentioned publications, if the sol-gel method is utilized as is, due to the heating temperature the fluorine contained in the thin film will evaporate resulting in a deterioration of water repellency.
Further, if the above sol-gel method is applied as it is to the resin substrate, as the water-repellent and abrasion resistant thin film is formed directly on the resin substrate, the water-repellent and abrasion resistant thin film does not firmly adhere to the resin substrate. Therefore, the resultant water-repellent and abrasion-resistant thin film is apt to exfoliate through long-term use, etc.
Also in the case of the acrylic-type cured coating formed on the resin substrate and the thin film of silicone-type cured coating formed on the resin substrate through the intermediation of a primer layer, the adhesion between the resin substrate and the acrylic-type cured coating and the adhesion between the resin substrate and the thin film of silicone-type cured coating formed thereon through the intermediation of a primer layer are rather poor and subject to exfoliation as in the above case. This is particularly true of the case in which the thin film is made of a silicone-type curable coating, which has a structure expressed as SiO.sub.2-x (x=0.6-0.7), that is, a structure which is different from the SiO.sub.2 structure of inorganic glass, so that the thin film is liable to be infiltrated by water and thereby swell. Thus, even if a silicone-type curable coating is applied to an acrylic-type curable coating, the resultant thin film is apt to exfoliate after being subjected to a hot-water test.
It is an object of the present invention to make it possible to easily form a thin film which has excellent abrasion resistance and excellent adhesiveness and which reliably exhibits water repellency on a resin substrate.
After careful examination on how to achieve this object, the present inventors have found that, by applying a mixed composition containing an organic-type polymerizing curable composition and a mixture containing silicone-type polymerizing curable composition and, as needed, a silane coupling agent, to a resin substrate and curing it thereon, it is possible to form a thin film which excels in abrasion resistance and water repellency, thus completing the present invention.